Methods and apparatus for synthesis of metal hydrides

ABSTRACT

An electrochemical process and apparatus for preparing metal hydride compounds from metal salts under a hydrogen atmosphere are disclosed. The electrochemical process may be integrated with chemical reaction of a boron compound to produce borohydride compounds. A metal salt and a borate are charged to the cathode of an electrolytic cell wherein the borate reacts with the hydride, to produce the borohydride compound.

This application claims the benefit of U.S. Provisional Application Ser.Nos. 60/622,789 filed on Oct. 29, 2004, and 60/662,555 filed on Mar. 17,2005, the entire disclosures of which are incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with Government support under CooperativeAgreement No. DE-FC36-04GO14008 awarded by the Department of Energy. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to the electrochemical reduction of activemetal salt compounds with applications in active metal hydride andactive metal borohydride production.

BACKGROUND OF THE INVENTION

Sodium borohydride is a very versatile chemical and is used in organicsynthesis, waste water treatment, and pulp and paper bleaching. The highhydrogen content of this compound also makes it a good candidate forbeing a hydrogen carrier, and it could play a major role as an enablerof a hydrogen economy if the cost of producing this chemical can bereduced.

Today, sodium borohydride is produced by the so-called Schlesingerprocess, which is a multi-step synthetic process, wherein sodiumborohydride is produced from the reaction of sodium hydride andtrimethyl borate in mineral oil. As none of the reagents are soluble inmineral oil, it is necessary to ensure high dispersions and the reactionmust proceed at elevated temperatures, typically around 250° C. Inaddition, mineral oil evaporates and can contribute to VOC emissions.

U.S. Pat. No. 3,734,842, U.S. Pat. No. 4,904,357, and U.S. Pat. No.4,931,154, the disclosures of which are incorporated by reference hereinin their entirety, refer to electrochemical synthesis of sodiumborohydride from aqueous sodium metaborate solution. Such processesinvolve conversion of sodium metaborate and water to form sodiumborohydride and oxygen in an electrical cell, as shown in the followinghalf-cell reactions:Cathode: B(OH)₄ ⁻+4H₂O+8e⁻→BH₄ ⁻+8OH⁻  (1a)Anode: 8O⁻⁻→4H₂O+2O₂+8e  (1b)

However, none of these processes has been implemented in commercialpractice.

BRIEF SUMMARY OF THE INVENTION

The invention is directed to electrochemical processes and apparatus forpreparing metal hydride compounds from active metal salts.

In accordance with one aspect of the present invention, molten activemetal salts are electrolyzed under a hydrogen atmosphere to produceactive metal hydrides.

In accordance with another aspect of the present invention, active metalsalts are electrolyzed in ionic liquids under a hydrogen atmosphere toproduce active metal hydrides.

In accordance with another aspect of the present invention, theelectrochemical process is integrated with a chemical reaction of aboron compound to produce boron hydride compounds.

In another aspect of the present invention, the electrochemical processis integrated with an in situ chemical reaction of an oxidized boroncompound to produce boron hydride compounds.

In another aspect of the present invention, oxidized boron compounds arereduced by reaction with active metal hydrides in a liquid salt toproduce boron hydride compounds.

These and other features and advantages of the invention will becomeapparent from the following detailed description that is provided inconnection with the accompanying drawings and illustrated exemplaryembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an electrolytic cell in accordance withone embodiment of the invention, where hydrogen-containing gas is passedinto the cathode compartment for synthesis of an active metal hydridefrom molten active metal salt;

FIG. 2 is a schematic diagram for producing borohydride anions accordingto an exemplary process of the present invention;

FIG. 3 is a schematic diagram for producing borohydride anions accordingto another exemplary process of the present invention; and

FIG. 4 is a view of an exemplary electrolytic cell suitable for use inthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an exemplary embodiment of the present invention, ametal salt or a mixture of metal salts are converted into a metalhydride via electrolysis in the presence of hydrogen. Without beinglimited by theory, it is thought that an electrochemical reduction ofthe metal salt yields metal at the cathode, and the metal formed thenreacts chemically with hydrogen to give metal hydride. The overallreaction is shown in Equation (2) wherein X represents a halide anion(the reaction product of the anion will depend on the anion, X, chosen),MX_(n)+n/2H₂→MH_(n)+n/2×2 (2)

where M is preferably selected from the group of metals and semimetalswherein the potential of the reaction between the metal, M, and oxygento make a metal oxide is greater than about 1.6 volts, where thepotential is defined as the negative of the free energy of reaction (AG,measured in joules per mole of metal) at standard conditions, whereintemperature is 298.15 K (25° C.) and pressure is 101.325 kPa (1 atm),divided by the number of moles of electrons transferred per mole ofmetal (n), divided by Faraday's constant (F) (Faraday's constant=96485coulombs/mole of electron), or Potential=−ΔG/nF; X is chosen from thegroup of anions comprising halides, tosylate, sulfate and sulfatederivatives, trifluoromethanesulfonate and other sulfonates, nitrate,phosphates, hexafluorophosphate, and other phosphate derivatives,phosphinates, dicyanamide, tetrafluoroborate, acetate, trifluoroacetate,borohydride, benzoate, tetrachloroaluminate, thiocyanate,thiosalicylate, tris(trifuoromethylsulfonyl)methide and other methides,and bis(trifluoromethylsufonyl)imide and other imides; and n is thevalence of the metal, preferably an integer from 1 to 4. Metals andsemimetals falling under this definition are herein referred to as“active metals.”

Active metals, include, but are not limited to, the alkali metals, thealkaline earth metals, transition metals from Groups 3, 4, 12, and thelanthanide family. The active metals form cations that include, but arenot limited to, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺ Sr²⁺, Ba²⁺,Sc³⁺, Ti³⁺, Ti⁴⁺, Zn²⁺, Al³⁺, Si⁴⁺, Y³⁺, Y⁺, Zr²⁺, Zr³⁺, Zr⁴⁺, Hf⁺,Hf³⁺, Hf³⁺, and lanthanides in the +3 oxidation state. M is preferablychosen from the group of alkali metals, and more preferably is lithium,sodium, potassium, and cesium; and X is preferably chloride or bromide.

An exemplary two-compartment electrolysis cell 100 employed in theprocess of the present invention is illustrated in FIG. 1. The cell 100comprises an anode compartment 104, anode 102, cathode compartment 112,cathode 108, separator 106 which separates the anode and cathodecompartments but allows ionic transport, and an optional gas inlet means110 to supply a gas comprising hydrogen to the cathode compartment. Theanodes and cathodes may comprise any suitable electrode material.

Separator 106 may preferably comprise a material such as glass, polymer,or ceramic that allows ionic transport between the cathodic and anioniccompartments, but restricts reaction between the active metal producedat the cathode and the product produced at the anode. Porous separatorssuch as porous glass, porous metal, porous plastics, and porous ceramicsare suitable separators. Paper, polymer, polymer membranes, andperfluoronated ion-conducting polymer membranes, are also suitableseparators. Nonlimiting examples of polymer separators includepolytetrafluoroethylene (PTFE), perfluoroalkoxy polymer,perfluorosulfonated ionomers, polyamides, nylon polymers, andpolyethylene. Optionally, cationic conducting ceramics may be employedas the separator. Nonlimiting examples of ceramic separators includelithium-β-aluminum oxide, lithium-β″-aluminum oxide,lithium-β/β″-aluminum oxide, lithium analogs of NaSICON ceramics,LiSICONs, and lithium ion conductors with perovskite structure,sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminumoxide, NaSICON ceramics, potassium-β-aluminum oxide,potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, andpotassium analogs of NaSICON ceramics.

In a preferred embodiment of the method of the present invention, one ormore active metal salts of formula MX_(n) are charged to the cathodechamber 112 to prepare a metal hydride. The cell is preferablymaintained at temperatures from about 70° C. to about 500° C., so thatthe one or more active metal salts are in a liquid molten state. Theactive metal salts can be used neat, i.e., without solvent, or a solventmay be included.

Alternatively, the one or more active metal salts can be dissolved in anionic liquid. Ionic liquids are defined herein as salts with a meltingpoint between about −100° C. and about 200° C., and preferablycontaining at least 1 carbon atom in the cation. Typical ionic liquidcations include, but are not limited to, mono-, di-, tri-, and tetrasubstituted ammonium; mono-, di-, tri-, and tetra substitutedphosphonium, N-alkylpyridinium, 1,3-disubstituted pyridiniums,1,4-disubstituted pyridiniums, 1,3-disubstituted imidazolium,1,2,3-trisubstituted imidazolium, 1,1 disubstituted pyrrolidiums,trialkylsulfonium, and trialkyloxonium cations. The anion in an ionicliquid can be any anion. Some typical anions are halides, but otherrepresentative and non-limiting examples include the group of commoncomplex ions such as tosylate, sulfate and sulfate derivatives,trifluoromethanesulfonate and other sulfonates, nitrate, phosphates,hexafluorophosphate, and other phosphate derivatives, phosphinates,dicyanamide, tetrafluoroborate, acetate, trifluoroacetate, borohydride,benzoate, tetrachloroaluminate, thiocyanate, thiosalicylate,tris(trifuoromethylsulfonyl)methide and other methides, andbis(trifluoromethylsufonyl)imide and other imides. Preferably, neitherthe cation nor the anion of the ionic liquid is easily reducible bystrong hydrides. It is not necessary that the liquid salt be a liquid atroom temperature, but only that at least a portion of the salt be liquidat the reaction temperature.

Hydrogen is preferably supplied to the cathode chamber as a gas streamvia a gas inlet means. Suitable gas inlet means for supplying a hydrogenor hydrogen-containing gas stream include a pipe, a sparger, a hose, ora hydrogen gas diffusion material. Alternatively, hydrogen can beabsorbed in a metal or metal alloy which can be released as thetemperature increases. Such metals or alloys can be impregnated withhydrogen and used as the cathode. Preferably, a gas stream comprisinghydrogen bubbles through or otherwise agitates the catholyte.

Upon the application of an electric potential, preferably from about 1.0to about 10.0 V, preferably from about 1.0 V to about 5.0 V, the activemetal ions are reduced at the cathode to the metal or semimetal as shownin Equation (3), and the anion is oxidized at the anode as shown inEquation (4a) for a monovalent anion such as a halide:Cathode: 2M^(y+)+ye⁻→M  (3)Anode: yX⁻→y/2X₂+ye⁻  (4a)

The active metal reacts with the hydrogen gas to form an active metalhydride compound as shown in Equation (5):M+y/2H₂→MH_(y)  (5)

In Equations (3), (4), and (5), y is an integer from 1 to 4 andtypically depends on the preferred (i.e. the most stable) oxidationstate of the active metal when it combines with oxygen to make theactive metal oxide. Some exceptions are known, such as titanium, whichpreferentially forms a TiH₂ hydride, rather than TiH₃ or TiH₄.

Hydrogen may be supplied to the anode as well as to the cathode toconvert the anion oxidation product to a desirable or valuable reactionproduct as shown in Equations (4b) and (4c) wherein X is a halide, andhalogen is converted to HX. Electrochemically oxidizing H₂ at the anodein preference to X⁻ will generally result in a lower cell potential thanthe comparable electrochemical system that generates X₂ depicted inEquation (4a).Anode: ½H₂→H⁺+e⁻  (4b)X⁻+H⁺→HX+e⁻  (4c)

In another aspect of the present invention, the electrochemical-chemicalprocess for obtaining metal hydrides according to the present inventioncan be incorporated into a process for producing boron hydridecompounds. In this embodiment, oxidized boron compounds are reduced by ahydride carrier in a liquid salt to produce a boron hydride compound.The hydride carrier may be, for example, derived from theelectrochemical reduction of active metal salts as described, wherein amolten active metal salt or mixture of molten active metal salts, eitherneat or in an ionic liquid, is converted into an active metal hydridevia electrolysis under an atmosphere of hydrogen. The process of thepresent invention provides a ready “one-pot” means to reduce boroncompounds such as boron-oxygen compounds and boron halide compounds, toboron hydride compounds including borohydride anions (BH₄ ⁻).

To produce a boron hydride compound, at least one active metal salt ischarged to the cathode compartment of an electrolytic cell and anelectric potential from about 1.0 V to about 10.0 V and preferably fromabout 1.0 V to about 5.0 V is applied to form the active metal asdescribed above.

After the active metal has reacted with hydrogen gas to produce theactive hydride, the applied potential may be removed and the cellmaintained at a temperature such that the metal hydride is at leastpartially dissolved in a liquid salt. Thus, for neat metal salt systems,the cell is maintained at temperatures above the melting point of theactive metal salt or mixture of active metal salts. For systems whereinthe metal salt was dissolved in an ionic liquid, the cell is maintainedat a temperature that allows the solvent to be liquid.

As shown schematically in FIG. 2, an oxidized boron species isintroduced to the liquid salt containing the active metal hydride. Theoxidized boron compound is selected from the group of boron oxygen andboron halide compounds. The boron-oxygen compound, collectively referredto as a “borate” in this application, is preferably selected from thegroup comprising trialkyl borates of formula B(OR)₃, where R is astraight-, branched-chain, or cyclic alkyl group containing from 1 to 6,preferably from 1 to 4, carbon atoms; boric oxide, B₂O₃; boric acid,B(OH)₃; and the group of alkali metal borate salts represented by theformula zM₂O.xB₂O₃.yH₂O, wherein z is ½ to 5; x is 0.1 to 5, y is 0 to10; and M is an alkali metal ion such as sodium, potassium, or lithium.The boron halide compounds can be chosen from the group of compoundsrepresented by formula BX₃, where X is a halide, preferably chloride orbromide.

The oxidized boron compound reacts with the active hydride. Equation(6a) illustrates the formation of borohydride from a trialkyl borate andan active metal hydride such as MH:4MH+B(OR)₃→MBH₄+3MOR  (6a)

The stoichiometry of the reduction reaction can be adjusted as shown inEquations (6b) to (6d) to ensure the generation of a borohydridecompound from various active metal hydrides and other oxidized boroncompounds:4MH₂+2B(OR)₃→M(BH₄)₂+3M(OR)₂  (6b)4MH₃+3B(OR)₃→M(BH₄)₃+3M(OR)₃  (6c)4MH₄+4B(OR)₃→M(BH₄)₄+3M(OR)₄  (6d)

Higher boron hydride compounds, such as diborane and triborohydridecompounds, can be prepared by varying the stoichiometric ratio betweenthe hydride and the oxidized boron compound, as illustrated in Equation(6e) for the formation of the diborane ion from a trialkyl borate:6MH+2B(OR)₃→6M⁺+⁻B₂H₆+6(OR)⁻  (6e)

The use of triborohydride compounds for hydrogen storage and relatedmethods for their preparation are described in co-pending U.S. patentapplication Ser. No. 10/741,192, entitled “Triborohydride Salts asHydrogen Storage Materials and Preparation Thereof,” filed Dec. 19,2003, the disclosure of which is incorporated by reference herein in itsentirety.

FIG. 3 schematically illustrates the process wherein the oxidized boroncompound is charged to the cathode compartment of an electrolytic cellalong with at least one active metal salt. For a neat reaction, themetal salt may be used as the “solvent” for the oxidized boron compoundby heating the cell at temperatures from about 70° C. to about 500° C.so that the one or more active metal salts are in a liquid molten state.Alternatively, the metal salt and oxidized boron compound may bedissolved in an ionic liquid.

The oxidized boron compound/alkali metal salt mixture is subjected to apotential from about 1.0 V to about 5.0 V to form the active metal asdescribed above. The oxidized boron compound reacts with the activehydride as it is produced, to form a boron hydride. In this case, thereactions illustrated in Equations (6a)-(6d) occur continuously as themetal hydride is formed.

Alkali metal borates, B₂O₃, and trialkyl borates such as B(OR)₃, may bereacted with alkali metal hydrides to obtain borohydride compounds insuspension. For example, at 275° C., NaH and B(OCH)₃ react in mineraloil to form NaBH₄, and NaOCH₃. The present invention can achieve thisreaction in a liquid salt, a solvent system that supports ionicconduction, and therefore electrochemical synthesis, wherein thereactants and products are in a dissolved state. The liquid saltsinclude molten active metal salts and ionic liquids.

As an example of the process of this exemplary embodiment of the presentinvention, borohydride anions are obtained from a molten mixture oflithium bromide, potassium bromide, and cesium bromide under a hydrogenatmosphere by the electrolytic process of the present invention, whereboric oxide is added to the melt before the application of a potential.Without being limited to any one particular theory, it is believed thatelectrolysis reduces the metal ions in the melt to the correspondingmetals, which then react with hydrogen to make the metal hydrides. Oneor more of the metal hydrides then react with the boric oxide to makeborohydride anions. A borohydride compound may then be isolated bysuitable separation and extraction steps.

For the particular case where the oxidized boron compound is sodiummetaborate, NaBO₂, it is preferable that the active metal hydride belithium hydride. Lithium hydride can be formed in situ according to theteachings herein by the electrolytic reduction of lithium bromide,either as a liquid molten salt or dissolved in an ionic liquid, under ahydrogen atmosphere to form lithium hydride.

Hydrogen may be supplied to the anode as well as the cathode to convertthe anion oxidation product and to lower cell potential according to theteachings herein.

In another embodiment of the invention, oxidized boron compounds areconverted to boron hydride compounds via reaction with metal hydridesdissolved in liquid salts, wherein the metal hydrides may be, forexample, commercially available products and/or not otherwise derivedfrom the electrochemical reduction of active metal salts as taughtherein. The metal hydrides should preferably be at least sparinglysoluble in the liquid salt solvent. The liquid salt may be a moltenmetal salt, or a mixture of molten metal salts, or an ionic liquid.

The metal hydrides may be selected from, for example, the group ofalkali metal hydrides, alkaline earth metal hydrides, aluminum hydridesincluding alane (AlH₃), and zinc hydride. A suitable metal hydride ischosen based on the standard reduction potential of the metal. Any metalwherein the standard reduction potential for the reaction of that metalwith oxygen to yield the most thermodynamically stable metal oxide ismore than about 1.6 V could be employed in this reaction.

The following examples further describe and demonstrate features of thepresent invention. The examples are given solely for illustration andare not to be construed as a limitation of the present invention.

EXAMPLE 1

A schematic illustration of the reactions taking place in the process isprovided in FIG. 2. The working electrode (cathode) is a nickel wire.The counter electrode (anode) is a platinum mesh inside a glass spargingtube. The interior of the glass sparger comprises the anode chamber, andthe region external to the sparging tube comprises the cathode chamber.

A mixture consisting of about 39.2 g LiBr, 18.1 g KBr, and 42.8 g ofCsBr was charged to cathode compartment and was electrolyzed at about 5V for about 5 hours under a hydrogen atmosphere to produce lithium metalat the cathode and bromine at the anode. The tube impeded mixing of thebromine that formed at the anode with the melt external to the tube, andthus slowed the back-reaction of lithium and bromine to lithium bromide.The tube also facilitated removal of gaseous bromine from the reactorunder a stream of flowing nitrogen. The reaction flask containing themelt was maintained in a constant temperature bath at about 300° C.

After about 5 hours, 587 mAh of current passed through the cell. Thenickel cathode and the sparging tube containing the anode were bothremoved from the melt, and a cold-water condenser was attached to thereaction flask. About 1.25 mL of tri-n-butyl borate was injecteddirectly into the melt using a syringe. The reaction was allowed toproceed for about 15 minutes, and the reaction flask was removed fromthe constant temperature bath and allowed to cool. The melt solidifiedas it cooled. The cool, solid melt was dissolved in 0.5 M NaOH aqueoussolution. A 50 mL sample of the solution was titrated using the iodateassay for borohydride as put forth in the Sodium Borohydride Digest byRohm and Haas Company. The titration indicated that 3.15×10⁻⁴ mol BH₄ ⁻was formed, a yield of 5.7% based on the 587 mAh of charge that passedthrough the cell. Boron NMR of the aqueous solution confirmed thepresence of borohydride anion (chemical Shift=−40.85 ppm, Splitting=80.6Hz).

EXAMPLE 2

Using the procedure described in Example 1, a melt consisting of about39.2 g LiBr, 18.1 g KBr, and 42.8 g of CsBr was electrolyzed under anargon atmosphere at about 3 V for 34 minutes. The potential, at 3 V, wastoo low to reduce the cations in the melt to metal, and instead reducedthe Ni surface of the cathode and generated bromine at the anode. Thereactor 400 was assembled as shown in FIG. 4. The reaction flask 402containing the melt in cathode compartment 414 was maintained in aconstant temperature bath at about 275° C. The working electrode(cathode) was a nickel frit 404 connected to an inlet 406 through whicha gas was passed (the gas could exit the reactor via outlet 420). Thecounter electrode 408 (anode) was a platinum mesh inside a glasssparging tube 410 with a glass frit separator 412. After 34 minutes,argon flowing through the cathode frit was replaced by flowing hydrogen.Argon continued to flow over the anode to remove bromine. The currentwas not interrupted as the gas changed. No changes were observed in thecurrent. After 74 minutes, about 0.2 grams of B₂O₃ was added to the meltin chamber 414. No appreciable changes in the current were observedafter being allowed to run an additional 126 minutes.

The electrolysis was reset to run for about 20 hours at about 5 V. After20 hours, 1975 mAh of current passed through the cell. The nickel fritcathode and the sparging tube containing the anode were both removedfrom the melt. The reaction flask was removed from the constanttemperature bath and allowed to cool. The melt solidified as it cooled,and the melt was dissolved in 0.5 M NaOH aqueous solution. A 50 mLsample of the solution was titrated using the iodate method forborohydride. The titration indicated that 2.34×10⁻⁴ mol BH₄ ⁻ anion wasformed, a yield of 1.3% based on the 1975 mAh of charge that passedthrough the cell. Boron NMR of the aqueous solution confirmed thepresence of borohydride anion.

EXAMPLE 3

A melt consisting of about 9.8 g LiBr, 4.5 g KBr, and 10.7 g of CsBrunder a nitrogen atmosphere was heated to about 250° C. To this melt,1.6 grams of B₂O₃ was added. With stirring, 0.27 g of LiH was added tothe melt. After adding LiH, the temperature bath was turned off, butstirring was continued until melt solidified. After dissolving thecooled melt in 100 mL of 0.5 M NaOH, a 50 mL sample of the solution wastitrated using the iodate method for borohydride. The titrationindicated that 5.3×10⁻³ mol BH₄ ⁻ was formed, a yield of 62% based onthe 0.27 grams of LiH added to the reactor. Boron NMR of the aqueoussolution confirmed the presence of the borohydride anion.

EXAMPLE 4

A mixture of about 39.2 g of LiBr, 18.1 g of KBr, and 42.8 g of CsBr,and 0.5 g of B₂O₃ were added to a 3-neck flask. The solids were heatedto about 300° C., a temperature at which this mixture is molten. Anickel metal sparging tube was inserted into the solution of moltenalkali bromides, and H₂ gas passed through the sparger and bubbledthrough the solution. This tube comprised the cathode. H₂ gas wasallowed to escape from the cell through one of the necks of the flask. Aglass tube terminating in a porous glass sparger was also inserted intothe solution. Platinum wire and platinum gauze were inside the tube, andthe platinum comprised the anode. The porous glass of the sparging tubeacted as a separator between the anode compartment (inside the glasstube) and the cathode compartment (outside the tube). The application ofabout 5 V of potential led to the passage of 1448 mAh of charge over 20hours.

The net reaction at the cathode was the generation of alkali metal andboron-hydride compounds. At the anode Br₂ gas was evolved. A stream ofAr gas helped carry the Br₂ gas out of the anode compartment.

After about 20 hours, the potential was removed, and the cathode spargerand the anode tube were withdrawn from the molten solution and thesolution was allowed to cool to room temperature and solidify. Theresulting solid was dissolved in about 100 mL of 0.5 M aqueous NaOHsolution. A small aliquot of solution was submitted to ¹¹B-NMR analysis,which showed the presence of borohydride (BH₄ ⁻) anions in solution.Another sample of the same aqueous solution was titrated, determiningthe yield of boron-hydride anions to be 4%, with respect to the numberof mAh of current passed through the cell.

EXAMPLE 5

Using the process described in Example 4, about 39.2 g of LiBr, 18.1 gof KBr, and 42.8 g of CsBr, and 1 g of B₂O₃ were added to a 3-neckflask. The solids were heated to about 300° C., a temperature at whichthis mixture is molten. A nickel metal sparging tube was inserted intothe solution of molten alkali bromides, and H₂ gas passed through thesparger and bubbled through the solution. This tube is the cell cathode.H₂ gas was allowed to escape from the cell through one of the necks ofthe flask. A glass tube terminating in a porous glass sparger was alsoinserted into the solution. Platinum wire and platinum gauze were insidethe tube, and the platinum comprised the anode. The porous glass of thesparging tube acted as a separator between the anode compartment (insidethe glass tube) and the cathode compartment (outside the tube). Theapplication of about 5 V of potential led to the passage of 1000 mAh ofcharge over about 6 hours.

After the 6 hour period, the potential applied across the anode andcathode was removed. The cathode sparger and the anode tube werewithdrawn from the molten solution and the solution was allowed to coolto room temperature and solidify. The resulting solid was dissolved inabout 100 mL of 0.5 M aqueous NaOH solution. A small aliquot of solutionwas submitted to ¹¹B-NMR analysis, which showed the presence of both BH₄⁻ (borohydride) and B₃H₈— (triborohydride) anions in solution. Anothersample of the same aqueous solution was titrated, determining the yieldof boron hydride anions to be 8.3%, with respect to the number of mAh ofcurrent passed through the cell.

EXAMPLE 6

Tetra-n-butylammonium bromide was heated to about about 120° C., andabout 1.5 mL of tri-n-butyl borate (B(O-Bu)₃) followed by about 0.5grams of sodium hydride was added to the hot ionic liquid. The startingmaterials are only sparingly soluble in the melt and fast stirring wasnecessary to ensure adequate dispersion. After addition of the sodiumhydride was complete, the melt was cooled to room temperature anddissolved in the minimum amount of aqueous 0.5 M NaOH. The presence ofborohydride in the aqueous solution was verified by NMR spectroscopy.

The above description and drawings illustrate preferred embodiments thatachieve the objects, features and advantages of the present invention.It is not intended that the present invention be limited to theillustrated embodiments. Any modification of the present invention thatcomes within the spirit and scope of the following claims should beconsidered part of the present invention.

1. A process for preparing a metal hydride compound, comprising:providing an electrolytic cell containing anode and cathode compartmentsseparated by a separator which is permeable to ions; supplying at leastone metal salt in molten form to the cathode compartment; applying anelectric potential to the cell; and providing hydrogen to the cathodecompartment.
 2. The process of claim 1, wherein the metal salt has theformula MX_(n), wherein M is an active metal cation selected from thegroup consisting of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺,Ba²⁺, Sc³⁺, Ti³⁺, Ti⁴⁺, Zn²⁺, Al³⁺, Si⁴⁺, Y³⁺, Y⁺, Zr²⁺, Zr³⁺, Zr⁺,Hf²⁺, Hf³⁺, Hf⁴⁺, and lanthanides in the +3 oxidation state; X is ananion selected from the group consisting of halides, tosylate, sulfate,sulfonates, nitrate, phosphates, hexafluorophosphate, phosphates,phosphinates, dicyanamide, tetrafluoroborate, acetate, trifluoroacetate,borohydride, benzoate, tetrachloroaluminate, thiocyanate,thiosalicylate, methides, and imides; and n is the valence of the activemetal cation.
 3. The process of claim 2, wherein M is selected from thegroup consisting of Li⁺, Na⁺, K⁺ and Cs⁺; and X is chloride or bromide.4. The process of claim 1, wherein the separator comprises a materialselected from the group consisting of lithium-β-aluminum oxide,lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide,sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminumoxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, andpotassium-β/β″-aluminum oxide.
 5. The process of claim 1, wherein theseparator is a NaSICON membrane.
 6. The process of claim 1, wherein theseparator is a LiSICON membrane.
 7. The process of claim 1, wherein theseparator comprises a material selected from the group consisting ofporous glass, porous metals, porous ceramics, porous plastics, paperpolymers, fluorinated polymers, ion-conducting polymers, and fluorinatedion-conducting polymers.
 8. The process of claim 1, wherein theelectrical potential is from about 1 to about 10 volts.
 9. The processof claim 7, wherein the electrical potential is from about 1 to about 5volts.
 10. The process of claim 1, further comprising passing hydrogenor a hydrogen containing gas to the cathode compartment through a gasinlet means.
 11. The process of claim 1, further comprising bubblinghydrogen gas through the cathode compartment to agitate the catholyte.12. The process of claim 1, further comprising providing hydrogen to thecathode compartment from hydrogen absorbed in a metal.
 13. The processof claim 1, further comprising providing hydrogen to the anodecompartment and electrooxidizing hydrogen at the anode.
 14. A processfor producing a metal hydride compound, comprising: providing anelectrolytic cell containing anode and cathode compartments separated bya separator which is permeable to ions; supplying at least one metalsalt to the cathode compartment, wherein the metal salt is at leastpartially dissolved in an ionic liquid; applying an electric potentialto the cell; and providing hydrogen to the cathode compartment.
 15. Theprocess of claim 14, wherein the metal salt has the formula MX_(n),wherein M is an active metal cation selected from the group consistingof Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺ Mg²⁺, Ca²⁺, Sr²⁺ Ba²⁺, Sc³⁺, Ti³⁺, Ti⁴⁺,Zn²⁺, Al³⁺, Si⁴⁺, Y³⁺, Y⁺, Zr²⁺, Zr³⁺, Zr⁴⁺, Hf²⁺, Hf³⁺, Hf⁴⁺, andlanthanides in the +3 oxidation state; X is an anion selected from thegroup consisting of halides, tosylate, sulfate, sulfonates, nitrate,phosphates, hexafluorophosphate, phosphates, phosphinates, dicyanamide,tetrafluoroborate, acetate, trifluoroacetate, borohydride, benzoate,tetrachloroaluminate, thiocyanate, thiosalicylate, methides, and imides;and n is the valence of the active metal cation.
 16. The process ofclaim 15, wherein M is selected from the group consisting of Li⁺, Na⁺,K⁺ and Cs⁺; and X is chloride or bromide.
 17. The process of claim 14,wherein the separator comprises a material selected from the groupconsisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide,lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminumoxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide,potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.
 18. Theprocess of claim 14, wherein the separator is a NaSICON membrane. 19.The process of claim 14, wherein the separator is a LiSICON membrane.20. The process of claim 14, wherein the separator comprises a materialselected from the group consisting of porous glass, porous metals,porous ceramics, porous plastics, paper polymers, fluorinated polymers,ion-conducting polymers, and fluorinated ion-conducting polymers. 21.The process of claim 14, wherein the electrical potential is from about1 to about 10 volts.
 22. The process of claim 21, wherein the electricalpotential is from about 1 to about 5 volts.
 23. The process of claim 14,further comprising passing hydrogen or a hydrogen containing gas to thecathode compartment through a gas inlet means.
 24. The process of claim14, further comprising bubbling hydrogen the cathode compartment toagitate the catholyte.
 25. The process of claim 14, further comprisingproviding hydrogen to the cathode compartment from hydrogen absorbed ina metal.
 26. The process of claim 14, further comprising providinghydrogen to the anode compartment and electrooxidizing hydrogen at theanode.
 27. The process of claim 14, wherein the ionic liquid is a saltcomprising a cation containing at least one carbon atom and having amelting point between about −100° C. to about 200° C.
 28. The process ofclaim 14, wherein the ionic liquid comprises a cation selected from thegroup consisting of mono-, di-, tri-, and tetra substituted ammonium;mono-, di-, tri-, and tetra substituted phosphonium, N-alkylpyridinium,1,3-disubstituted pyridiniums, 1,4-disubstituted pyridiniums,1,3-disubstituted imidazolium, 1,2,3-trisubstituted imidazolium, 1,1disubstituted pyrrolidiums, trialkylsulfonium, and trialkyloxoniumcations.
 29. The process of claim 14, wherein the ionic liquid comprisesan anion selected from the group consisting of halides, tosylate,sulfate, sulfonates, nitrate, phosphates, hexafluorophosphate,phosphates, phosphinates, dicyanamide, tetrafluoroborate, acetate,trifluoroacetate, borohydride, benzoate, tetrachloroaluminate,thiocyanate, thiosalicylate, methides, and imides.
 30. A process forproducing a boron hydride compound, comprising: providing anelectrolytic cell containing anode and cathode compartments separated bya separator which is permeable to ions; supplying at least one metalsalt in molten form to the cathode compartment; applying an electricpotential to the cell; providing hydrogen to the cathode compartment;and providing a boron compound to the cathode compartment.
 31. Theprocess of claim 30, wherein the metal salt has the formula MX_(n),wherein M is an active metal cation selected from the group consistingof Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sc³⁺, Ti³⁺,Ti⁴⁺, Zn²⁺, Al³⁺, Si⁴⁺ Y³⁺, Y⁺, Zr²⁺, Zr³⁺ Zr⁴⁺, Hf²⁺, Hf³⁺, Hf⁴⁺, andlanthanides in the +3 oxidation state; X is an anion selected from thegroup consisting of halides, tosylate, sulfate, sulfonates, nitrate,phosphates, hexafluorophosphate, phosphates, phosphinates, dicyanamide,tetrafluoroborate, acetate, trifluoroacetate, borohydride, benzoate,tetrachloroaluminate, thiocyanate, thiosalicylate, methides, and imides;and n is the valence of the active metal cation.
 32. The process ofclaim 31, wherein M is selected from the group consisting of Li⁺, Na⁺,K⁺ and Cs⁺; and X is chloride or bromide.
 33. The process of claim 30,wherein the separator comprises a material selected from the groupconsisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide,lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminumoxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide,potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.
 34. Theprocess of claim 30, wherein the separator comprises a material selectedfrom the group consisting of NaSICON and LiSICON membranes.
 35. Theprocess of claim 30, wherein the separator comprises a material selectedfrom the group consisting of porous glass, porous metals, porousceramics, and porous plastics.
 36. The process of claim 30, wherein theelectrical potential is from about 1 to about 10 volts.
 37. The processof claim 36, wherein the electrical potential is from about 1 to about 5volts.
 38. The process of claim 30, further comprising passing hydrogenor a hydrogen containing gas to the cathode compartment through a gasinlet means.
 39. The process of claim 30, further comprising bubblinghydrogen the cathode compartment to agitate the catholyte.
 40. Theprocess of claim 30, further comprising providing hydrogen to thecathode compartment from hydrogen absorbed in a metal.
 41. The processof claim 30, wherein the boron compound is an oxidized boron compound.42. The process of claim 41, wherein the oxidized boron compound issodium metaborate and the metal halide is lithium bromide.
 43. Theprocess of claim 30, further comprising maintaining the cell at atemperature of about 70 to about 500° C.
 44. The process of claim 30,further comprising providing hydrogen to the anode compartment andelectrooxidizing hydrogen at the anode.
 45. The process of claim 30,wherein the electric potential is removed before providing the boroncompound.
 46. The process of claim 30, wherein the boron compound isprovided before applying the electric potential.
 47. The process ofclaim 30, further comprising separating the boron hydride compound. 48.The process of claim 30, wherein the boron compound is a boron halide.49. The process of claim 30, wherein the boron compound is an alkylborate.
 50. The process of claim 30, wherein the boron compound is aborate.
 51. The process of claim 30, wherein the boron compound isselected from the group consisting of boric oxide and boric acid. 52.The process of claim 30, wherein the boron compound is an alkali metalborate salt.
 53. A process for producing a boron hydride compound,comprising: providing an electrolytic cell containing anode and cathodecompartments separated by a separator which is permeable to ions;supplying at least one metal salt to the cathode compartment, whereinthe metal salt is at least partially dissolved in an ionic liquid;applying an electric potential to the cell; providing hydrogen to thecathode compartment; and providing a boron compound to the cathodecompartment.
 54. The process of claim 53, wherein the metal salt has theformula MX_(n), wherein M is an active metal cation selected from thegroup consisting of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺ Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺,Sc³⁺, Ti³⁺, Ti⁴⁺, Zn²⁺, Al³⁺, Si⁴⁺, Y³⁺, Y⁺, Zr²⁺, Zr³⁺, Zr⁴⁺, Hf²⁺,Hf³⁺, Hf⁴⁺, and lanthanides in the +3 oxidation state; X is an anionselected from the group consisting of halides, tosylate, sulfate,sulfonates, nitrate, phosphates, hexafluorophosphate, phosphates,phosphinates, dicyanamide, tetrafluoroborate, acetate, trifluoroacetate,borohydride, benzoate, tetrachloroaluminate, thiocyanate,thiosalicylate, methides, and imides; and n is the valence of the activemetal cation.
 55. The process of claim 54, wherein M is selected fromthe group consisting of Li⁺, Na⁺, K⁺ and Cs⁺; and X is chloride orbromide.
 56. The process of claim 53, wherein the separator comprises amaterial selected from the group consisting of lithium-β-aluminum oxide,lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide,sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminumoxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, andpotassium-β/β″-aluminum oxide.
 57. The process of claim 53, wherein theseparator comprises a material selected from the group consisting ofNaSICON and LiSICON membranes.
 58. The process of claim 53, wherein theseparator comprises a material selected from the group consisting ofporous glass, porous metals, porous ceramics, porous plastics, paperpolymers, fluorinated polymers, ion-conducting polymers, and fluorinatedion-conducting polymers.
 59. The process of claim 53, wherein theelectrical potential is from about 1 to about 10 volts.
 60. The processof claim 53, wherein the electrical potential is from about 1 to about 5volts.
 61. The process of claim 53, further comprising passing hydrogenor a hydrogen containing gas to the cathode compartment through a gasinlet means.
 62. The process of claim 53, further comprising bubblinghydrogen the cathode compartment to agitate the catholyte.
 63. Theprocess of claim 53, further comprising providing hydrogen to thecathode compartment from hydrogen absorbed in a metal.
 64. The processof claim 53, wherein the ionic liquid is a salt comprising a cationcontaining at least one carbon atom and having a melting point betweenabout −100° C. to about 200° C.
 65. The process of claim 53, wherein theionic liquid comprises a cation selected from the group consisting ofmono-, di-, tri-, and tetra substituted ammonium; mono-, di-, tri-, andtetra substituted phosphonium, N-alkylpyridinium, 1,3-disubstitutedpyridiniums, 1,4-disubstituted pyridiniums, 1,3-disubstitutedimidazolium, 1,2,3-trisubstituted imidazolium, 1,1 disubstitutedpyrrolidiums, trialkylsulfonium, and trialkyloxonium cations.
 66. Theprocess of claim 53, wherein the ionic liquid comprises an anionselected from the group consisting of halides, tosylate, sulfate,sulfonates, nitrate, phosphates, hexafluorophosphate, phosphates,phosphinates, dicyanamide, tetrafluoroborate, acetate, trifluoroacetate,borohydride, benzoate, tetrachloroaluminate, thiocyanate,thiosalicylate, methides, and imides.
 67. The process of claim 53,wherein the boron compound is a boron-oxygen compound.
 68. The processof claim 53, wherein the boron compound is an alkyl borate.
 69. Theprocess of claim 53, wherein the boron compound is a borate.
 70. Theprocess of claim 53, wherein the boron compound is selected from thegroup consisting of boric oxide and boric acid.
 71. The process of claim53, wherein the boron compound is an alkali metal borate salt.
 72. Theprocess of claim 53, wherein the boron compound is sodium metaborate andthe metal halide is lithium bromide.
 73. The process of claim 53,further comprising maintaining the cell at a temperature of about 70 toabout 500° C.
 74. The process of claim 53, further comprising providinghydrogen to the anode compartment and electrooxidizing hydrogen at theanode.
 75. The process of claim 53, wherein the electric potential isremoved before providing the boron compound.
 76. The process of claim53, wherein the boron compound is provided before applying the electricpotential.
 77. The process of claim 53, further comprising separatingthe boron hydride compound.
 78. A process for producing borohydrideanions comprising dissolving a metal hydride and a boron compound in atleast one liquid salt.
 79. The process of claim 78, wherein the metalhydride is selected from the group consisting of alkali metal hydrides,alkaline earth metal hydrides, aluminum hydrides and zinc hydrides. 80.The process of claim 78, wherein the metal hydride comprises a metalcharacterized in that the standard reduction potential for the reactionof the metal with oxygen is at least about 1.6 V.
 81. The process ofclaim 78, wherein the metal hydride is formed by the steps of: supplyingat least one active metal salt in molten form to a cathode compartmentof an electrolytic cell containing anode and cathode compartmentsseparated by a separator which is permeable to ions; applying anelectric potential to said cell to reduce the metal compound at thecathode; and passing hydrogen or a hydrogen containing gas in thecathode compartment while the compound is reduced at the cathode. 82.The process of claim 78, wherein the metal hydride is formed in situ.83. The process of claim 78, wherein the boron compound is an oxidizedboron compound.
 84. The process of claim 78, wherein the liquid salt isa molten active metal salt.
 85. The process of claim 78, wherein theliquid salt is a mixture of molten active metal salts.
 86. The processof claim 78, wherein the liquid salt is an ionic liquid.
 87. The processof claim 78, wherein the liquid salt is a mixture of at least one moltenactive metal salt and at least one ionic liquid.
 88. An apparatus forreducing boron compounds to produce boron hydride compounds, comprising:an anode compartment containing on anode; a cathode compartmentcontaining a cathode; a separator between the anode and cathodecompartments, wherein the separator is permeable to ions; at least oneinlet for charging metal salt and boron compounds to the cathodecompartment; and a means for supplying hydrogen to the cathodecompartment.
 89. The apparatus of claim 88, wherein the apparatus isconfigured to maintain the cathode compartment at a temperature of about70° C. to about 500° C.
 90. The apparatus of claim 88, wherein theseparator comprises a material selected from the group consisting oflithium-β-aluminum oxide, lithium-β″-aluminum oxide,lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminumoxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide,potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.
 91. Theapparatus of claim 88, wherein the separator is a NaSICON membrane. 92.The apparatus of claim 88, wherein the separator is a LiSICON membrane.93. The apparatus of claim 88, wherein the separator comprises amaterial selected from the group consisting of porous glass, porousmetals, porous ceramics, porous plastics, paper polymers, fluorinatedpolymers, ion-conducting polymers, and fluorinated ion-conductingpolymers.
 94. The apparatus of claim 88, further comprising a means forbubbling hydrogen gas through the cathode compartment to agitate thecatholyte.
 95. The apparatus of claim 88, wherein the cathodecompartment contains hydrogen absorbed in a metal adapted to releasehydrogen when heated.